There have recently been proposed various wiring boards including printed-circuit boards having wiring patterns formed on substrates.
Wiring boards for use as actuator or sensor devices having piezoelectric/electrostrictive layers have a wiring pattern comprising an electrode layer formed by printing, for example, on a ceramic substrate and another wiring pattern comprising a piezoelectric/electrostrictive layer and an electrode layer on the other wiring pattern.
Wiring boards having piezoelectric/electrostrictive layers can be used as actuator or sensor devices by supplying an electric signal to the wiring pattern and applying an electric field to the piezoelectric/electrostrictive layer. Specifically, the actuator device operates to displace the piezoelectric/electrostrictive layer by applying an electric field to the piezoelectric/elctrostrictive layer, and the sensor device operates to extract an electric signal generated in response to a pressure applied to the piezoelectric/electrostrictive layer, through the wiring pattern.
In the wiring boards, since the bonding strength between the wiring pattern and the ceramic substrate or between the wiring pattern and the piezoelectric/electrostrictive layer is weak, the wiring pattern may peel off while the wiring board is being machined, e.g., cut off or polished, or being cleaned, e.g., ultrasonically, at the boundary between the ceramic substrate and the wiring pattern or at the boundary between the wiring pattern and the piezoelectric/electrostrictive layer.
Depending on the planar configuration of the wiring pattern, the wiring board may contain a region where no electrode layer is formed on the ceramic substrate. In such a region, the ceramic substrate and the piezoelectric/electrostrictive layer are disposed in confronting relation to each other, and a void will be produced after the wiring board is fired. The reason for this problem is that the material of the ceramic substrate and the material of the piezoelectric/electrostrictive layer are hardly joined to each other.
The void makes the piezoelectric/electrostrictive layer float partly off the ceramic substrate. Since the floating portion of the piezoelectric/electrostrictive layer is not positionally secure with respect to the ceramic substrate, it can freely move under applied external forces, and hence is liable to peel off.
To prevent the piezoelectric/electrostrictive layer from peeling off, when the wiring board is cut off, for example, any load applied to the piezoelectric/electrostrictive layer, e.g., a force imposed on the piezoelectric/electrostrictive layer when the wiring board is severed, should be small. It is thus necessary to limit the cutting process to small load conditions, and the limited cutting process takes a long period of time and results in a low throughput.
When the wiring board is cleaned, any load applied to the piezoelectric/electrostrictive layer, e.g., a force imposed on the piezoelectric/electrostrictive layer when the wiring board is cleaned, should also be small. Therefore, in order to remove smears from the wiring board, a long period of time is required to clean the wiring board, and the number of man hours involved in the cleaning process is increased.
When a piezoelectric/electrostrictive layer is peeled off, the following drawbacks are caused:
(1) The functions of the wiring board are lowered;
(2) If a wiring pattern is peeled off a piezoelectric/electrostrictive layer, then the electrostatic capacitance is reduced, resulting in a reduced amount of strain in the piezoelectric/electrostrictive layer. Therefore, the displacement of the piezoelectric/electrostrictive layer is reduced;
(3) If a wiring pattern is peeled off a ceramic substrate, then the strain generated in the piezoelectric/electrostrictive layer is less transmitted to the ceramic substrate. Therefore, the displacement of the piezoelectric/electrostrictive layer is reduced; and
(4) The overall mechanical strength of the wiring board is reduced, resulting in a reduction in the resonant frequency of the wiring board when the board is used as an actuator.